Recently, the use of mobile information devices including smartphones and wireless terminals such as mobile healthcare terminals has been rapidly increasing. Such wireless terminal devices are usually supplied with power by using a secondary battery embedded in the devices. Generally, since the capacity of the secondary battery is small, the secondary battery cannot be used for a long time with a single charge, and accordingly, it is inconvenient to periodically charge the secondary battery.
Generally, a wired charging method using a DC adapter is commonly used for charging. However, in this case, it is inconvenient for a user to directly connect a wireless terminal to a wired adapter. In particular, since a wearable device such as a smart watch is small, a connection work for wired charging is more difficult. In addition, in the case of wired charging, since a plurality of wired charging adapters have to be used for simultaneously charging a plurality of batteries or loads, charging cost increases and charging becomes complicated. Therefore, there is a need for a method of simultaneously charging various wireless terminal devices in a simple manner without using a plurality of adapters.
Recently, magnetic coupled resonant wireless power transfer technology has attracted attention. The magnetic coupled resonant wireless power transfer technology basically transmits power by using time-varying magnetic field coupling at a short distance. An independent self-resonator having a high quality factor (Q-factor) of several tens or more is used in a transmission/reception unit, or a self-inductance of a transmission/reception self-resonant coil increases and a resistance decreases, so that a quality coefficient of a transmission/reception coil has at least several tens or more. Based on this technology, it is possible to improve an effective power transmission distance that can be recharged, and it is possible to perform free positioning charging to any load existing within a certain range in space. In particular, the magnetic coupled resonant wireless power transfer technology has an advantage that can simultaneously transmit power to multiple loads.
However, when power is transmitted to multiple loads, an optimum impedance matching condition is changed according to the position and the charging state of the load. Accordingly, when the optimum impedance matching condition is changed, the power transfer efficiency for each load is deteriorated. In addition, in the case of multiple loads, a charging current or voltage for each load is changed according to a state of a battery. Thus, a need for a method of supplying power required by the individual battery as appropriately as possible is required for efficiently charging each battery. For example, in a case where two loads are simultaneously charged by one input, if a large amount of power is required in one load and a relatively small amount of power is required in the other load, it is more preferable to adjust the supply of power to each load by taking into account the state of each load.
Therefore, there is a need for a method that can optimize the supply of power to a plurality of loads, for example, by appropriately distributing power to each load by controlling the supply of power to each load by taking into account the state of each load such as a battery, and further, efficiently charging a plurality of batteries, but a suitable alternative has not been proposed.